[SYCL][Fusion] Add missing event waits

[sommerlukas]

Add missing event::wait for the event returned by queue::submit while in fusion mode.

The event returned from queue::submit is associated with the internal placeholder command. It depends on the event returned by complete_fusion, but is not identical to it, so an additional wait is required in the tests to avoid the assert to fail.

[sommerlukas]

This should fix the failures encountered in #1554.

[sommerlukas]

The following tests failed in the pipeline:

Failed Tests (3):
SYCL :: ESIMD/api/svm_gather_scatter_scalar_off.cpp
SYCL :: ESIMD/dword_atomic_cmpxchg_scalar_off.cpp
SYCL :: ESIMD/dword_atomic_smoke_scalar_off.cpp

These should be unrelated to this PR.

@steffenlarsen: I couldn't find a matching issue/PR, should I report this?

[steffenlarsen]

Given the failures are compiler errors in untouched tests that don't show up in newer PRs, I have a suspicion this is just a test-change ahead of a new nightly coming in, so let us merge this without an issue and see if it pops up again later.